I get an error when I do tojson in flutter - flutter

Question

I tried tojson in flutter, but I get the following error:
[VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: type 'List<Tag>' is not a subtype of type 'String' in type cast
this is my code
import 'package:flutter_traveling/model/Tag.dart';
class Diary {
String? title;
String? location;
String? address;
List<Tag>? tagList;
List<Pages>? pages;
String? date;
Diary({
this.title,
this.location,
this.tagList,
this.pages,
this.date,
this.address,
});
Diary.fromJson(Map<String, dynamic> json) {
title = json['title'] ?? "";
location = json['location'] ?? "";
address = json["address"] ?? "";
tagList = json['tag_list'] ?? <Tag>[];
pages = json['pages'] ?? <Pages>[];
date = json['date'] == null ? '' : json['date'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['title'] = this.title;
data['location'] = this.location;
data['tag_list'] = this.tagList;
data['pages'] = this.pages;
data['date'] = this.date;
data['address'] = this.address;
return data;
}
}
class Pages {
int? order;
String? description;
List<String>? images;
Pages({
this.order,
this.description,
this.images,
});
Pages.fromJson(Map<String, dynamic> json) {
order = json['order'] ?? 0;
description = json['description'] ?? "";
images = json['images'] == null ? [] : json["images"].cast<String>();
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['order'] = this.order;
data['description'] = this.description;
data['images'] = this.images;
return data;
}
}
class Tag {
int? id;
String? tagName;
Tag({this.id, this.tagName});
Tag.fromJson(Map<String, dynamic> json) {
id = json['id'];
tagName = json['tag_name'] ?? "";
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['id'] = this.id;
data['tag_name'] = this.tagName;
return data;
}
}
I don't know what causes this error to occur. please help
My guess is that when I run tojson, there seems to be a problem with the type, but I don't know which part and how to fix it, I'm just starting out with flutter, any advice would be appreciated.
i got err when change code like this.
data['tag_list'] = this.tagList.toString();
data['pages'] = this.pages.toString();
error is
Unhandled Exception: type 'String' is not a subtype of type 'List<Tag>?'

Answer 1

Try to replace
tagList = json['tag_list'] ?? <Tag>[];
with
tagList = List<Tag>.from(json["tag_list"].map((x) => Tag.fromJson(x)))
And this line
data['tag_list'] = this.tagList;
with
data['tag_list'] = this.tagList.map((tag) => tag.toJson()),

Answer 2

you should call the toJson() methods from your tags as well:
data['tag_list'] = this.tagList.map((tag) => tag.toJson()),